The ribosome is the universal site of protein synthesis, containing some of the most highly conserved of all biological sequences. Nevertheless, the ribosome is robust to mutation, capable of functioning when challenged with base or amino acid substitutions in its highly conserved functional centers. As major targets of antibiotics, these functional centers are the sites of numerous antibiotic-resistance mutations. While it has been well established that antibiotic-Resistance mutations carry a substantial fitness cost, the structural basis for this burden is only now within the scope of our technical ability to investigate. In this proposal, we describe a synthetic approach using genetics, chemical probing and X-ray crystallography of ribosomes from the thermophilic bacterium Thermus thermophilus to address the structural robustness of ribosome active sites and its relationship to biological fitness. Our development of T. thermophilus ribosome genetics has enabled us to identify or construct antibiotic-resistant mutants at will. We also now have the technical ability to crystallize wild-type and mutant 30S ribosomal subunits and 70S ribosomes and to determine their three-dimensional structures by X-ray diffraction. Together with the development of novel chemical probing techniques to investigate RNA conformational dynamics, these abilities have placed us in a unique position to address three specific issues. The first aim of our proposal is to use streptomycin-resistance mutations as a paradigm for examining the mutational robustness of a conserved ribosome functional center that participates in global conformational changes of the 30S subunit. Our second aim is to use tuberactinomycin-resistance to examine the effects of mutations on the structure and function of an intersubunit bridge that is critical for large-scale rotational motions of the entire 70S ribosome. The third aim is to use deleterious antibiotic-resistance mutations in the peptidyltransferase active site to evolve compensatory mutations that restore fitness, and to examine their structural effects using X-ray crystallography. The goal of this aim is to detect as yet unrecognized long-range functional relationships throughout the ribosome. We will also use the peptidyltransferase active site to examine the limits of robustness of ribosome functional centers to mutation. In addition to providing a more complete mechanistic understanding of antibiotic resistance at an unprecedented level of resolution, these efforts are directed towards establishing fundamental principles of ribosome structural organization and evolution.